This invention relates to semiconductor devices and, in particular, to a semiconductor device with a multilayer contact and a method for making such an apparatus.
Semiconductor devices, such as transistors, can consist of multiple layers of semiconducting or insulating materials. In a vertical transistor there are at least three layers including an emitter, base and collector. Charge carriers, either holes or electrons or both, pass from the emitter, through the base, to the collector. A metallic contact is deposited on each of the layers for operating the transistor.
Horizontal or field effect transistor are also multilayer devices. The source, gate and drain each have a metallic contact on a common, upper surface. The gate may include a Schottky barrier that creates a depletion layer to control the transfer of charges that flow underneath the gate from the source to the drain.
Recent developments in semiconductors have produced multilayer heterojunction devices. These devices, either vertical or horizontal transistors, are typically fashioned from alternate layers of gallium arsenide (Ga As) and aluminum gallium arsenide (Al Ga As). Under voltages suitably applied to such a device, a thin, high mobility region is formed at the junction of such materials. This region, in a horizontal transistor, is termed a two dimensional electron gas or 2 DEG. Electrons that enter a 2DEG can move very quickly. Such quick motion gives such transistors a switching speed almost four-times as fast as other transistors.
In horizontal high mobility devices, such as field effect transistors, it is known to place a source and drain contact on a common upper surface. A Schottky barrier, as a gate, is positioned between the source and drain and on the upper surface. The gate controls the flow of charges in the 2 DEG between the source and drain by depleting the 2 DEG beneath the gate electrode. See, for example, U.S. Pat. Nos. 4,558,337; 4,471,367 and 4,455,564. The source and drain contacts are both on the upper layer and do not extend to other layers.
Vertical high mobility devices are more difficult to fabricate. They comprise very thin layers that have to be precisely deposited or etched to expose lower layers for forming ohmic contacts. Two hundred angstroms is typical of the thickness of some layers. Molecular beam epitaxial deposition equipment can deposit such thin layers. However, it is difficult to fix metal contacts on devices with such thin layers. Moreover, it is desirable in some applications e.g. oscillators, to switch the conduction path between different layers. As such, it would be desirable to have a device with a metallic contact selectively connectable to two or more layers, i.e. a multilayer contact. Such a contact could extend from an upper surface through to a desired lower layer. Such a contact would reduce the need for precise etching of intermdiate, thin layers to expose the contact layer for receiving the metal contact.